1. Field of the Invention
The present invention generally relates to an information recording medium such as an optical disc, an optical card and the like, and more particularly to a multi-layer-information-recording medium which has a plurality of recording layers layered on spacer layers.
2. Description of the Related Art
In recent years, optical discs are widely used as means for recording and reproducing data such as video data, audio data, computer data and the like. A high density recording disc called DVD (Digital Versatile Disc) has been used in practice. As one type of DVD, there is a multi-layer disc in a laminate structure which has a plurality of recording layers that can be read from one side of the disc. A two-layer disc having two recording layers on one side has been used in practice as a disc dedicated to reproduction.
As illustrated in FIG. 1, the two-layer DVD dedicated to reproduction comprises a shallow recording layer, which is the first layer viewed from the side on which data is read, i.e., closer to a light incident side surface, and a deep or second recording layer. With the two-layer disc, any signal recorded in the shallow recording layer and the deep recording layer can be read from one side of the disc only by moving the focus of a reproducing light beam. The shallow recording layer is made of a translucent film such that a light beam can transmit the shallow recording layer and read a signal from the deep recording layer, and the film thickness and material are chosen conveniently for the shallow recording layer. A reflective film is used for the deep recording layer. An optically transparent spacer layer having a high transmittance at the wavelength of light is disposed between the shallow recording layer and the deep recording layer in order to separate these layers by a constant distance.
The DVD standard defines that a transparent cover layer on a recording layer of a single-layer disc, having only one recording layer, 600 μm thick, as illustrated in FIG. 2. On the other hand, a two-layer disc is formed to have a first recording layer and a second recording layer positioned at depths of 570 μm and 630 μm from the surface on which a light beam is incident, i.e., above and below the depth of 600 μm at which the recording layer of a single-layer DVD is disposed. The positioning of the two layers above and below the single recording layer in the thickness direction is employed in the two-layer disc because an optical pickup system for recording and reproducing signals conforming to the DVD standard comprises an objective lens having a relatively small numerical aperture of 0.6 which is designed for the cover layer of 600 μm thick, and even with such an objective lens having a small numerical aperture, a deviation of approximately 30 μm in depth of the first layer and the second layer each from the single recording layer does not significantly affect the reading of signals. In this event, though the deviation of 30 μm of the recording layers causes wave aberration in a reading light beam, the amount of wave aberration is too small to cause a problem when the numerical aperture is on the order of 0.6.
A long program such as a movie which overflows the first recording layer of the two-layer disc is reproduced from the two recording layers. The DVD standard also defines a single side signal-reproducing scheme, called an opposite track path scheme, for continuously reproducing from two layers. The opposite track path scheme involves reproducing from the recording layer at a depth of 570 μm from the inner periphery to the outer periphery, jumping the focus from the outer periphery of this recording layer to the recording layer at a depth of 630 μm, and reproducing signals on the deeper recording layer from the outer periphery to the inner periphery. In this event, by reading an information region representing the contents of the disc recorded on the layer at a depth of 570 μm, the apparatus can sense the title of the DVD, a program duration, or the two-layer disc in accordance with the opposite track path scheme.
Meanwhile, an increasing amount of information requires a higher density for next-generation optical disc. It is considered that the numerical aperture of the objective lens be increased to 0.8 or more for a higher density. When using an objective lens having such a large numerical aperture, the amount of wave aberration caused by an error in thickness of the cover layer on the recording layer increases too much to read signals, thereby failing to readily reproduce a next-generation optical disc when it is in a two recording layer structure. It is therefore considered that an optical system capable of adjusting the amount of wave aberration should be incorporated in a pickup to make a compensation for preventing the wave aberration in accordance with the depth of a recording layer.
When an optical system for compensating for the wave aberration is used to read a single-layer disc and a multi-layer disc such as a two-layer disc of the next-generation while maintaining the compatibility, the difference exists in thickness of a cover layer corresponding to depths between the respective recording layers, so that a light beam must be focused on each recording layer while correcting the same or the wave aberration to search for lead-in information and the like. This give rise to a problem that the time taken for starting reproduction becomes long if a two-layer disc is reproduced immediately after a single-layer disc was reproduced. In addition, with a large numerical aperture, a larger thickness of the cover layer causes an allowable range to be significantly narrowed down for the inclined disc, so that a multi-layer disc which has a larger thickness of cover layer than a single-layer disc must be fabricated with an improved planarity for the surface of the disc more than the single-layer disc.